<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #box {
            width: 300px;
            height: 200px;
            border: 1px solid black;
        }
    </style>
</head>

<body>
    <div id="app">
        <div id="box">
            {{mystr.toUpperCase()}}
        </div>
        <div>
            <input v-model="mystr" type="text" placeholder="请输入小写字母">
        </div>

    </div>

    <script src="./js/vue.js"></script>
    <script>
        var app = new Vue({
            el: "#app",
            data: {
                mystr: ""
            }
        })
    </script>
</body>

</html>